The present invention relates to micro-lithography and, in particular, relates to resolution enhancement and proximity correction features utilized in the field of photolithography for semiconductor device fabrication.
Until recently, improvements in the resolution of optical lithography processes have come largely from the use of Deep Ultra Violet (DUV) exposure sources having shorter exposure wavelengths (xcex). Currently available DUV exposure sources include the following two types of excimer lasers: (1) Krypton Fluoride (KrF) having a xcex of 248 nm and (2) Argon Fluoride (ArF) having a xcex of 193 nm. However, in order to fabricate device generations at 0.18 xcexcm (180 nm) and below, it has become clear to the industry that the lithography process will need to resolve feature dimensions below xcex if either of the two foregoing exposure sources are to be utilized.
An alternative to utilizing the foregoing excimer lasers is non-optical lithography using very short exposure wavelength sources such as Extreme Ultra Violet (EUV), X-ray, or electron beam (E-beam). Unfortunately, all the non-optical lithography technologies require some form of technological break-through combined with an adequate support infrastructure in order to become xe2x80x9cproduction-worthy,xe2x80x9d or in other words, commercially feasible. While it is likely that the necessary technology break-through and infrastructure build-up will eventually occur, to date they have not. As such, for semiconductor device production having design rules in the range of 0.18 xcexcm down to 0.10 xcexcm, optical lithography is currently considered to be the most economical and preferred process technology. Accordingly, there exists a need to find innovative methods that can consistently pattern sub-xcex device features with optical lithography.
For sub-xcex device features, the mask pattern image formation is strongly dependent on the optical diffraction with the immediately adjacent patterns. For binary-type masks such as chrome patterns on a quartz glass substrate, the resolution is diffraction limited as imposed by the exposure tool. However, by introducing a xcfx80 phase shift as the exposure wavefront passes through the mask patterns, it has been demonstrated that the optical resolution limit can be greatly extended. Depending on the degree of phase shifting effect on the mask, it is possible to double the spatial frequency resolution for the mask patterns. In other words, the pattern resolution achievable with a phase shift mask (PSM) can reach xc2xdxcex.
The first PSM application in optical lithography was reported by M. D. Levenson in 1982 (IEEE Trans. Electron. Devices 29, 1828, 1982). Since this time, there has been a continuous effort in the industry to explore and develop PSM technology. However, due to the inherent complexity of mask design, the learning curve for making and applying PSM has been long and arduous. Nonetheless, several forms of practical PSM technology have been developed. With regard to making line and space patterns, there are three major types: 1) alternating PSM (as originally proposed by Levenson); 2) attenuated PSM; and 3) chromeless PSM.
In accordance with alternating PSM, 0-phase and xcfx80-phase alternating areas are formed between chrome mask features. There are two major unsettled issues concerning alternating PSM design. The first is the unavoidable conflict of phase assignment, and the second is the unwanted resist patterns caused by the 0 to xcfx80 phase transitions on the mask. The currently proposed solutions to these issues either add more complexity to the mask design or require the use of more than one mask. As such, none of the proposed solutions are attractive from a xe2x80x9ccommercializationxe2x80x9d or xe2x80x9cproduction costxe2x80x9d point of view.
From a design viewpoint, alternating PSM is much more challenging than attenuated PSM. Attenuated PSM typically utilizes an energy absorbing thin film layer deposited on a quartz substrate. This energy absorbing film has the property of causing a 180 degree (xcfx80) phase change in the electric (E) field as the exposure wavefront passes through the mask. After mask patterns have been delineated, there is a xcfx80 phase shift in between the attenuated film areas and non-patterning glass only areas. Unlike the traditional chrome mask, this type of PSM typically causes some amount of attenuation by the actinic (or effective) exposure xcex. The extent of attenuation is mainly dependent on the phase shifting film structures and/or the interlayer thin chrome film deposited on the glass substrate. The attenuation permits a certain percent of actinic exposure xcex to xe2x80x9cleakxe2x80x9d through the phase shifting areas of the mask. Normally the amount of attenuation is described as percent transmission (% T).
FIGS. 1A-1C illustrate aerial images of a typical attenuated PSM with intensity profiles (% T) equaling 100%, 25% and 5%, respectively. As can be observed from FIGS. 1A-1C, relatively high intensity levels result from the attenuated, phase shifted areas. The strength of the intensity levels seems to be related to the % T. Specifically, the higher the % T, the stronger the intensity level. For the non-phase shifting (glass only) areas, the intensity levels remains unchanged. In order to minimize these xe2x80x9cundesirablexe2x80x9d intensity levels resulting from the attenuation, the standard industry practice is to limit the % T to be at most 5% for DUV exposure xcex.
Finally, with regard to chromeless PSM, the xcfx80 phase shift area can be made by simply etching into the quartz substrate to a precise depth. The non-etched areas and the etched areas have an optical path difference (OPD) that can cause a xcfx80 phase shift as the exposure wavefront passes through the mask. Optically, the chromeless PSM concept is substantially an extension of the attenuated PSM. In other words, the chromeless PSM can be thought of as an attenuated PSM with 100% transmission. As observed in FIG. 1A, for a high % T, the xe2x80x9cleakagexe2x80x9d of actinic exposure xcex causes very strong aerial image intensity profiles.
Heretofore, the standard method for controlling the xe2x80x9cundesirablexe2x80x9d intensity levels is to limit the % T. Unfortunately, very low % T limits the potential resolution advantage that can otherwise be gained by using the phase shifting film. The lower the % T, the more the resulting film acts like a non-phase shifting chrome film. Accordingly, in order to achieve higher resolution, it is much more desirable to use a high % T attenuated PSM. One solution to the foregoing problem is to utilize an opaque film layer to xe2x80x9cblockxe2x80x9d off the leaky phase shifting areas.
As shown in FIG. 2, a chrome opaque film can effectively minimize the xe2x80x9cundesirablexe2x80x9d intensities. The width of the chrome blocking layer needs to be smaller than the high % T attenuated phase shifting areas. To manufacture this chrome blocking layer, it is necessary to perform a second resist coating, alignment, and imaging process. This second step requires tight control of the width of the chrome blocking layer and the alignment margin in order to ensure the chrome blocking layer will be effective and not interfere with the phase shifting pattern areas.
It is clear that one disadvantage of using a chrome blocking layer is the need to perform two alignment processes for making such a reticle. The chrome blocking layer is normally imaged by an optical laser pattern generator. As such, it often suffers from lower resolution and limited alignment accuracy. In addition, this second process step adds to both the complexity and cost of the mask.
Moreover, as stated, the chrome blocking layer is utilized to xe2x80x9cblockxe2x80x9d the xe2x80x9cundesirablexe2x80x9d aerial image intensities formed by the high % T phase shifting areas. As shown in FIG. 2, the remaining aerial images are mainly formed by the non-phase shifting patterns. However, the aerial image intensity levels are not as high as the ones formed by the high % T phase shifting areas. As a result, the expected resolution enhancement from the traditional chrome-blocked PSM is substantially negated.
Accordingly, there remains a need for a mask which allows for the use of the high intensity levels formed by the high % T phase shifting areas (because the higher intensity levels offer an inherent higher resolution potential), and which does not require the use of chrome blocking layers so as to reduce the overall complexity and cost of the mask.
Accordingly, it is an object of the present invention to provide a cost effective and practical method for patterning sub-0.25 xcex resist line features using a type of 100% transmission, xe2x80x9cattenuatedxe2x80x9d PSM.
More specifically, the present invention relates to a method for making a mask for optically transferring a lithographic pattern corresponding to an integrated circuit from the mask onto a semiconductor substrate by use of an optical exposure tool. The method comprises the steps of de-composing the existing mask patterns into arrays of xe2x80x9cimaging elements.xe2x80x9d These imaging elements are xcfx80-phase shifted and are separated by a non-phase shifting and sub-resolution element referred to as anti-scattering bars (ASB). In essence, the ASBs are utilized to de-compose the larger-than-minimum-width mask features to form xe2x80x9chalftone-like imaging patterns. The placement of the ASBs and the width thereof are such that none of the xcfx80-phase shifting elements are individually resolvable, but together they form patterns substantially similar to the intended mask features. The isolated minimum width line features can be formed by a single xcfx80-phase imaging element.
As described in detail below, the method of the present invention provides important advantages over the prior art. Most importantly, the present invention discloses a method for patterning sub-0.25 xcex resist line features using a type of 100% transmission, xe2x80x9cattenuatedxe2x80x9d PSM. In accordance with the present invention, instead of trying to eliminate the image intensity caused by high transmission xcfx80-phase pattern areas, the method of the present invention makes use of the high contrast aerial image to achieve excellent printing resolution.
In addition, by extending the concept of ASBs, it is possible to xe2x80x9cdecomposexe2x80x9d the xcfx80-phase feature patterns. Using the decomposed xcfx80-phase shifting elements, it is possible to reconstruct the random shaped device patterns, while simultaneously performing optical proximity correction by manipulating the size, shape, and placement of the decomposed xcfx80-phase shifting elements.
Furthermore, as the imaging concept for the high transmission, attenuated PSM method of the present invention is very similar to the conventional, non-phase shifting chrome mask patterning methods, it is believed that the adoption of this technology by the industry will be much easier as compared to alternating PSM technology. From the mask layout point of view, by utilizing the method of the present invention there is no need to be concerned with avoiding phase conflicts and printing of phase transitions onto the wafer. Thus, the mask layout complexity is greatly reduced. Moreover, as there is no need to use an opaque chrome blocking layer, the mask making process is much simpler.
The method of present invention also provides for the decomposition of a minimum line feature into an array of xcfx80-phase shifting elements so that it is possible to use a wider dimension element on the 4X mask to achieve printing 0.25 xcex feature on a wafer. As a result, the mask used for printing the sub-0.25 xcex features can be made at a reasonable cost.
Additional advantages of the present invention will become apparent to those skilled in the art from the following detailed description of exemplary embodiments of the present invention.
The invention itself, together with further objects and advantages, can be better understood by reference to the following detailed description and the accompanying drawings.